In mounting structures in which an electronic component is surface-mounted to a wiring substrate, the heat resistance of the electronic component itself has been improved, but the overall heat resistance, in particular, durability to withstand repeatedly applied thermal shock, of the mounting structures including solder cannot yet be considered as sufficient. With the improvement of the functions of automobiles, there is a need to realize a motor that includes a mounting structure and can be used in the engine compartment of an automobile or its vicinity, including, for example, a motor for driving valves such as a slot valve and an inlet valve. To realize this, it is necessary that no cracks occur in solder joint portions of the mounting structure, after performing a thermal shock test designed assuming the use in the engine compartment, namely, a 1000-cycle thermal shock test from −40 to 150° C. It is also necessary that lead-free solder is used in order to reduce the environmental load.
Patent Literature 1 describes Sn—Ag—Bi—In-based lead-free solder that has excellent mechanical properties and can be used for substrate mounting.
Patent Literature 2 states that a land to which the electrode of an electronic component is connected via solder, and a wiring pattern formed in a substrate are connected with wiring having a narrow width, in order to prevent cracks from occurring in solder due to heat stress.
Patent Literature 3 describes a mounting structure using Sn—Cu-based or Sn—Ag-based lead-free solder. The literature states that a metal core substrate with excellent thermal conductivity and a ceramic substrate with excellent heat resistance can be used as a substrate.